The present invention relates to a method of manufacturing glass optical elements by molding glass materials with high surface precision, including lenses in which one or both surfaces are concave. In particular, the present invention relates to a method permitting the manufacture of glass optical elements by molding glass materials with high surface precision, including lenses having a ratio b/a of the peripheral thickness b to the center thickness a of at least 1.5.
Methods in which softened glass is pressed with a forming mold having a shape identical or similar to that of a product to directly manufacture lenses without grinding or polishing (high-precision pressing methods) are widely utilized. Since high-precision pressing permits the manufacturing of large quantities of lenses of both spherical and aspherical surfaces at low price, it is employed to manufacture the optical system lenses of a variety of optical equipment and products such as digital cameras and video cameras. In recent years, demands have been made of high-precision pressing to manufacture lenses that are large and difficult to mold.
However, in lenses in which one or both surfaces are concave and in lenses in which there is a large difference in thickness between the center and the edge (rim) portions of the lens, once molded surfaces are transferred to the glass by the pressing process, the transfer surface often deforms during cooling and removal from the forming mold or during annealing, making it impossible to obtain a lens of desired surface shape. This is particularly acute in lenses having two concave surfaces with a large difference in thickness between the center and edge portions.
Japanese Unexamined Patent Publication No. Hei 7-267658 describes a method of molding concave meniscus lenses in which a glass material in a softened state is pressed to obtain a glass optical element wherein the pressed glass is maintained at a certain temperature at or above the glass transition temperature for a prescribed time in a cooling step. While this method is principally directed to removal of the thermal stress that has been generated in the glass material within a short period, it does not permit the molding of lenses with desired surface shapes that are particularly hard to achieve, which are those the present invention focuses upon.
Accordingly, an object of the present invention is to provide a method of manufacturing glass optical elements such as glass lenses by press-molding glass materials with high precision, including lenses that are concave on one or both surfaces and have a large b/a ratio, where a denotes center thickness and b denotes peripheral thickness.
Research conducted by the present inventors resulted in the discovery that by subjecting a molded product in the form of a lens to a prescribed pressure within a certain temperature range after the press-molding and up to the removal from the forming mold, the above-stated problem was solved; the present invention was devised on this basis.
The present invention relates to a method for manufacturing a glass optical element comprising steps of:
molding a glass material softened by heat with a forming mold which comprises an upper mold and a lower mold, each having a molding surface so that optically functional surfaces are formed on the glass material by applying a molding pressure,
cooling the glass material so that the glass material obtains a predetermined viscosity, and
removing the cooled glass material from the forming mold,
wherein a temperature of the glass material is maintained, in the cooling step, within a range of (Tg+30) to (Tgxe2x88x9250) degrees centigrade at least for a predetermined time, and a secondary pressure is applied to the glass material at least during the predetermined time, so that the strain in the glass material is reduced,
where Tg represents glass transition temperature of the glass.